(1) Field of the Invention
The present invention relates to a catalyst for purification of exhaust gas, containing an alkali metal and/or an alkaline earth metal as an NOx-adsorbing component, particularly a catalyst for purification of an exhaust gas emitted from an internal combustion engine of gasoline vehicle, diesel vehicle or the like.
(2) Description of Related Art
In recent years, regulation for exhaust gas has become stricter, and lean burn engines, direct injection engines, etc. have come into wide use. In this connection, NOx-adsorbing catalysts capable of effectively purifying the NOx present in ah exhaust gas, in a lean atmosphere have been put into practical use. As the NOx-adsorbing component used in the NOx-adsorbing catalyst, there are known alkali metals such as K, Na, Li, Cs and the like; alkaline earth metals such as Ba, Ca and the like; and rare earth elements such as La, Y and the like. Recently, it has been attempted to use, in particular, K which is superior in NOx adsorption at high temperatures.
NOx-adsorbing catalysts are generally constituted by loading a catalyst layer containing the above-mentioned NOx-adsorbing component, on a carrier composed of an oxide type ceramic (e.g. cordierite) or a metallic material (e.g. Fexe2x80x94Crxe2x80x94Al alloy). Such a carrier, however, has a problem; that is, it is easily corroded and deteriorated by alkali metals or part of alkaline earth metals, particularly, Li, Na, K and Ca when they become active under the high temperatures of exhaust gas.
In view of the above problem of the prior art, the present invention aims at providing a catalyst for purification of exhaust gas, obtained by loading, on a carrier, a catalyst layer containing an alkali metal and/or an alkaline earth metal as an NOx-adsorbing component, which catalyst can suppress the deterioration of the carrier caused by the alkali metal and/or the alkaline earth metal and therefore can be used over a long period of time.
According to the present invention, there is provided a catalyst for purification of exhaust gas comprising: a carrier, and a catalyst layer containing an alkali metal and/or an alkaline earth metal, the catalyst layer being loaded on the carrier, in which catalyst the carrier-composing material contains Si in an amount of 0.5 to 10.0% by weight as expressed as an oxide.